Mark Swan, of Deer Lane, is being held on $501,000 bond at the Troop A barracks in Southbury and is expected to appear in state Superior Court in Danbury on Tuesday.

Police began their investigation after the 19-year-old alleged victim went to the resident trooper's office in New Fairfield on Saturday and reported that Swan had assaulted her on Wednesday.

Swan had been the woman's case worker since she was 16, and an investigation "supported the victim's claim" that an assault occurred, police said.

Authorities took Swan into custody shortly before midnight Saturday, when a trooper stopped a vehicle that was observed being driven erratically on Route 39 near Bogus Hill Road and found him behind the wheel.

Department of Children and Families spokesman Gary Kleeblatt said Sunday that Swan will be placed on administrative leave pending a departmental investigation and could face disciplinary action "up to and including termination."

Investigators gave no indication whether the sex between Swan and the victim was consensual or part of a continuing relationship, but under state law, a person can be charged with second-degree sexual assault when the victim is under the age of 18 and the accused is either a guardian or is responsible for their general supervision or welfare, or the victim is in custody of the law and the accused has supervisory or disciplinary authority over her.

Kleeblatt confirmed that the victim was "a current client" of Swan's, but said other information about Swan's employment history with the agency wasn't available on Sunday.

A state police spokesman said Sunday that since state offices are closed Monday for Presidents Day, no further information on the case would be released prior to Swan's scheduled appearance in court.

Police said the case remains under investigation by the resident state trooper, the State Police Western District Major Crime Squad and the Department of Children and Families.